Due to the high probability of errors associated with wireless communication networks and systems, methods are employed for retransmitting data when a receiver detects an error in the transmitted data. One such method is an Automatic Repeat Request (ARQ), examples of which include stop-and-wait ARQs and selective repeat ARQs.
The Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, conformance and interoperability with which is promoted by the Worldwide Interoperability for Microwave Access (WiMAX) Forum, have adopted a standardized retransmission method known as Hybrid ARQ (HARQ) that is used in the Medium Access Control (MAC) layer. According to the HARQ method, when the receiver detects errors in a transmission, data from the original transmission is stored by the receiver and retransmission is requested. The receiver then combines the original transmission and one or more subsequently received copies of the original transmission to effectively increase the signal quality. There is a higher probability that one or more copies of the original transmission will be successfully decoded without errors than if the copy of the transmission had not been combined with the original transmission or the previously received copy. This is known as combining gain and is also employed in other retransmission methods to improve performance.
Each HARQ transmission may contain latency sensitive data, such as voice data packets and/or latency tolerant data packets, typically in the form of Service Data Units (SDUs). If the latency sensitive data is not successfully transmitted within a predetermined time, it is considered “stale” and may be discarded. In contrast, the latency tolerant data should be retransmitted until transmission is successful.
One drawback of retransmission methods including the HARQ method is that each retransmission adds latency. Detecting an error in the received transmission and requesting retransmission typically takes a minimum of 20 ms. Therefore, one retransmission will typically add another 20 ms or more to the total latency the data experiences and this latency problem is exacerbated with each retransmission required for successful transmission.
Hence, where both latency sensitive data and latency tolerant data are transmitted and an error is detected, the transmitter must determine whether it is worthwhile to retransmit an identical copy of the original transmission, thus taking advantage of combining gain from one or more previous retransmissions, or whether it should transmit an entirely new transmission, where the original latency sensitive data is omitted. Transmitting an entirely new transmission results in a smaller transmission, but all combining gain is lost because combining gain only works if identical transmissions are retransmitted. Currently, there are no well defined methods for determining when to transmit a copy of the original transmission and when to generate and send a new transmission.
Skilled addressees will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the relative dimensions of some of the elements in the figures may be distorted to help improve understanding of embodiments of the present invention.